For purposes of illustration the invention will be mainly exemplified with reference to the fluid feed stock commonly encountered in the papermaking industry.
The stock used in papermaking ordinarily consists of fibres, which are often wood fibres but may also be of cotton or other materials, in water suspension. It commonly happens that these wood fibres in water suspension are contaminated by the presence of undesirable particles, which may include sand, metal filings, and the like, but which also may include bark particles from the original log, incompletely pulped particles from knots in the wood, and the like.
In the paper industry, it is well-known to use what are called in that industry "centrifugal cleaners" or, more simply, "cleaners" to separate undesirable particles from the good pulp. These are devices of the type otherwise more generally known as "liquid cyclones," or sometimes as "hydrocyclones."
The centrifugal cleaners used in papermaking stock preparation ordinarily consist of a closed, hollow, slightly truncated, inverted cone, of metal, ceramic, plastic, or other rigid material, having a closed base of the cone at the upper end, provided with an inlet pipe or inlet orifice or orifices, which enter tangentially, at the top, near the cone base, and into which a dilute suspension of papermaking stock is pumped under pressure by an external pump connected by suitable piping; and additionally provided with an axial exit pipe or "accept pipe" from the center of the base of the cone, which accept pipe projects axially inward for a short distance into the interior of the hollow cone to form the so-called "vortex finder"; and additionally provided with an axial opening or orifice sometimes called the "reject tip" at the apex of the cone, which, as usually mounted, is at the lower end of the cone.
In the operation of centrifugal cleaners, the stock is pumped continuously into the device through the tangential inlet, under some considerable pressure, entering at relatively high velocity, and rotates rapidly within the hollow cone, from which two streams emerge, the larger fraction of the incoming stock leaving by the accept pipe through the cone base, and called the "accept flow" or "accepts", while the smaller fraction, called "reject flow" or "rejects", leaves by way of the reject tip in the apex of the cone. The rapid spinning or spiralling motion of the stock within the hollow cone causes the suspended particles to be subjected to large radial acceleration forces, or centrifugal force. Since the useful and desirable fibres for papermaking have densities or specific gravities very close to that of the suspending water, and since the contaminating particles are frequently of greater density, the heavy contaminating particles tend to be thrown outward toward the wall of the cone, where they travel in a spiral path downward toward the reject tip, in the so-called "dirt lamella", immediately adjacent to the wall of the cone. A large proportion of the heavy particles leave with the reject flow through the reject tip. The accept flow contains a substantially reduced concentration of the undesirable particles of higher density, compared with the incoming flow or "feed flow".
Centrifugal cleaners, as presently used in the industry, are characterized by the fact that a relatively high reject flow must be maintained for their proper operation. While it is typical that the undesirable high density particles in the incoming feed flow comprise less than one percent of the total weight of particles in the feed flow, good operation, and good removal of the said high density particles from the accept flow can nevertheless only be obtained if the proportion of solid particles in the reject flow is fifteen percent to twenty percent, or even more, of the weight of particles in the feed flow. Therefore, an undesirably high percentage of good fibre is rejected with the undesirable particles in the reject flow.
Considerable research has been done on centrifugal cleaners over many years by various investigators, and there is an extensive technical literature on the subject. The said research has established that despite the mechanical simplicity of the device, there is a quite complex flow pattern of the stock, or fluid, within a centrifugal cleaner when it is in operation. Specifically, it may be noted here that there exists in all operating centrifugal cleaners, as a component of the abovementioned complex flow pattern, what has been called the "leakage flow." There is a small continuous flow radially inward across the underside of the closed top of the device, which is the base of the cone, down the outside of the vortex finder to the open end of the said vortex finder, and thence out the accept pipe. This fraction of the flow through the device, the leakage flow, comprises some fraction of the incoming feed flow which passes through the device without being subjected to any substantial radial acceleration, or centrifugal force, and which thus does not have the undesirable contaminant particles removed from it. Furthermore, it is understood by those skilled in the art, that no changes in the geometric configuration of the device, such as change in the dimensions, diameter, cone angle, or shape, size, or surface configuration of the interior surface of the cone base or of the vortex finder, can entirely eliminate the leakage flow. It is recognized that the leakage flow is characteristic of centrifugal cleaners as presently used, because it is induced in accordance with the well-known laws of fluid mechanics, by viscous drag and fluid friction typical of the flow of all real fluids past a solid boundary, which is sometimes spoken of as the "boundary layer effect".
Thus it may be stated that, in all centrifugal cleaners heretofore used in the papermaking industry, which is to say those devices in which the accept flow leaves the device axially through the cone base, with or without the use of a vortex finder, the separation of undesirable particles can never be complete, because of the existence of the leakage flow.